Wood-based materials (for example, plywoods (veneer board, etc.), particle boards, fiber boards (medium density fiberboard: MDF, etc.), and laminated woods) are generally produced by applying or spraying an adhesive etc. onto wood-based elements (raw materials) (for example, various sizes of fibers, small pieces, and veneers obtainable by finely dividing woods or herbaceous plants), followed by molding the wood-based elements through pressurizing and heating. The wood-based materials are natural regenerable (or renewable) materials, in which stabilities of the strength and the size are enhanced and thus defects peculiar to woods are removed while utilizing advantages of the woods. From the viewpoint of the protection of the earth environment, the protection of workers producing the wood-based materials, and the prevention of sick house syndrome, there have been developed, as the adhesive to be used, an aqueous adhesive which does not cause diffusion of formaldehyde and contains no organic solvent.
When the wood-based material (for example, particle board) is produced using a urea resin and a phenol resin, a mixture of a wood-based element with an adhesive is generally heated at a temperature of about 130 to 170° C. and molded. Therefore, it is preferable that an aqueous adhesive is also heated to approximately the same temperature, thus making it possible to produce the wood-based material. However, when using the aqueous adhesive, higher temperature is often needed.
There is also a need that the wood-based material thus obtained (for example, particle board) is excellent in performances such as bending strength, bending strength under wet condition, water-absorption thickness expansion coefficient, and peeling strength. However, when using the aqueous adhesive, the performances are often unsatisfactory.
Patent Literature 1 discloses an aqueous binder comprising a reducing saccharide such as dextrose, and an ammonium carboxylate such as triammonium citrate (see CLAIMS and Table 1 in [0131], etc.). This aqueous binder is employed to produce a fiber glass and a wood-based fiber board (see [0016] to [0017], etc.). However, since the fiber glass and the wood-based fiber board of Patent Literature 1 have a lower bending strength under wet condition and a higher water-absorption thickness expansion coefficient, it cannot be said that they are suited for a structural material which is required to have severe moisture resistance.
Patent Literature 2 discloses a bonding composition comprising a saccharide (sucrose, maltose, etc.) and a polycarboxylic acid (citric acid, malic acid, maleic anhydride, polymaleic acid, polyacrylic acid, etc.) in which formaldehyde is hard to diffuse (see Claims 1 and 2, [0031] to [0032], etc.). Patent Literature 2 further discloses production of a wood-based board using the composition (see [0126], etc.). Patent Literature 2 discloses that the bonding composition comprises a thickener (see Claims 4 and 13, [0046], etc.), and the bonding composition comprising a thickener is specifically disclosed in Table 5.
The bonding composition of Patent Literature 2 comprises the polycarboxylic acid to improve joint power between woods. However, the wood-based board of Patent Literature 2 also has an unsatisfactory bending strength under wet condition and an unsatisfactory water-absorption thickness expansion coefficient, and is not suited for a structural material which is required to have severe moisture resistance.